(a) Field of the Invention
The present invention relates generally to digital communications and, more particularly to a method and apparatus for (i) backhauling compressed voice in a digital communication system without additional degradation in voice quality, thereby reducing the number of leased PCM trunks and (ii) improving voice quality on calls that involve multiple voice compressed links using similar voice compression technology such as mobile-to-mobile calls in cellular or mobile satellite systems.
(b) Description of Related Art
The most common form of digital modulation used in communications is pulse code modulation (PCM). PCM represents a message as a sequence of coded digital pulses. PCM is used in communication systems to transfer voice or data from one location to another via a communication link, which may include a wire link such as an E1 line or a wireless link such as a satellite communication link. Typically, PCM is used to represent an analog signal in digital form. In the communications industry, PCM is used to refer to a digital communication link that operates at a 64 kilobits per second (kbps). There are eight thousand samples of an analog signal taken every second, and each sample is represented by eight bits, hence 64 kbps.
Bandwidth conservation is a concern in almost all communication applications. In a digital satellite link, for example, a 64 kbps signal is compressed and transmitted at 4 kbps to save bandwidth. For example, a subscriber unit in a communication system may sample a user's voice at a rate equivalent to 64 kbps or more, however, that signal is compressed into a signal that is 4 kbps before it is uplinked to a satellite. The satellite receives the signal from the subscriber unit and rebroadcasts it to a gateway. The gateway receives the downlinked signal from the satellite and decompresses the signal back to a 64 kbps signal. The compression and decompression of a signal in such a manner mildly compromises the integrity of the signal. However, each successive signal compression and decompression introduces more noise into the signal.
In many satellite communication systems the gateway decompresses the signals to 64 kbps and couples them to leased terrestrial E1 transmission lines that eventually connect with a conventional PSTN network. The number of E1 lines needed is proportional to the bandwidth that must be carried from the gateway to the PSTN network. In many applications the leased E1 lines may span many hundreds of miles and may traverse the boarders of many countries. The lease rates for E1 lines are based on required bandwidth and line length. Additionally, traversal of national borders may increase the cost to lease a particular E1 line. All of these line leasing costs are typically passed on to the user placing the telephone call.
The cost associated with leasing E1 lines for hauling traffic between a gateway (or base station) to a point of presence (POP) in a national or international network can be readily appreciated. Accordingly, it would be desirable to have a method and apparatus for reducing the costs associated with the leasing of the E1 lines in a satellite communications.
It is possible to perform one more (possibly different) compression and decompression steps on the information to be sent across E1 lines to achieve cost reduction. However, the overall voice quality degradation can be easily noticeable and will be worse than the worst performance of the two compression and decompression schemes. A similar problem exists in many satellite and cellular systems when voice calls are established between two satellite users (using two satellite hops) or two cellular users. The heart of the present invention is to achieve cost reduction and, at the same time, avoid voice quality degradation.